Excretory and reproductive organs, Lab 5

Excretory organs remove the end products of metabolism and excess water. In unicellular organisms the contractile vacuole does this; flatworms have protonephridia (tubes with flame cells), annelids and molluscs metanephridia (ciliated funnels) in each segment, crustaceans the green gland, and insects and arachnids Malpighian tubules, which excrete uric acid as crystals and save water. In vertebrates the kidney is linked to the circulation: a pronephros in fish and amphibian larvae, a mesonephros in adult fish and amphibians, and a true kidney (metanephros, with many nephrons) in reptiles, birds and mammals. In the embryo the excretory and reproductive systems develop from mesoderm and are linked. In sexual reproduction cnidarians make gametes from ectodermal cells, flatworms are hermaphrodites, roundworms and arthropods have separate sexes. In fish and amphibians fertilisation is mostly external, in reptiles internal; they lay eggs with a thick shell rich in food, birds also lay eggs, while in mammals the embryo develops in the uterus and the young are fed with milk.